1999 chevy Monte Carlo, 87,000 miles.
I have a
p0420 on auto zone scanner, they said it could be a misfire or oxygen sensor.
What could be problem. Thanks.                          

Hi. A code p0420 is catalytic converter inefficient. The converter is going to have to be replaced. If
no other codes, then that should take care of the problem. If other codes, will need to get those
repaired first as that may be why the converter has failed.
Other related questions

Problem with my 2003 Chevy Monte Carlo SS
3800 V6 Series 2

Repairs made = replaced ECM
tip/donation yes
tech = kk
question = car cranks but stalls immediately. Can hold gas pedal down a   little and get it to stay
cranked but runs rough, blows black smoke and stinks really bad. Removed sensor connections to
see if check engine light would come on while cranked but it did not. Was told to do this to possibly
conclude the computer was bad. Replaced computer and
relearned password per instructions but have same problem since computer was replaced.
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Chevy Monte Carlo P0420- Cranks And Stalls